In subterranean well treatment operations, high viscosity fluids are often formulated using dry additives which are mixed with water or other aqueous fluids at the job site. Such commercial mixing procedures are known to involve inherent problems, particularly on remote sites or when large volumes of fluid are required. For example, special equipment for mixing dry additives in water is required and problems such as chemical dusting, uneven mixing, lumping of gels while mixing and extended preparation and mixing time are involved. The mixing and physical handling of large quantities of dry chemicals require a great deal of manpower and, when continuous mixing is required, the accurate and efficient handling of dry chemicals is extremely difficult. Furthermore, with respect to batch mixing applications, the job delays can result in the deterioration of pre-mix gels and the potential loss thereof as well as chemical losses due to tank bottoms and problems associated with the cost of pre-treatment tank clean-up.
More recently, gelable materials have been supplied in a non-aqueous slurry concentrate which is useful in continuous processes supplying a viscous, gelled aqueous fluid for subterranean well treatment operations. Such a slurry concentrate typically comprises a polymer slurry wherein a hydratable polymer is dispersed in a hydrophobic solvent in combination with a suspension agent and a surfactant and, possibly, including other additives commonly employed in well treatment applications. The hydratable polymer inherently disperses even in the oil-based fluid. This feature tends to eliminate lumping and premature gelation problems and tends to optimize the initial dispersion of the hydratable gel when added to water. However, the rate of hydration of a polymer is still a critical factor particularly in continuous mix applications wherein the necessary hydration and associated viscosity rise must take place over a relatively short time span corresponding to the residence time of the fluid during the continuous mix procedure.
Hydration is a process by which a hydratable polymer chemically combines with water to create a viscous gel. Once the polymer is dispersed, its ability to absorb water will dictate hydration or hydration rate. Several factors determine how readily the polymer will hydrate or develop viscosity such as the pH of the system, the amount of mechanical shear applied in the initial mixing phase, the concentration of salts in the aqueous fluid and the polymer loading in the system. Hydration rate can be influenced through pH control agents which may be blended with the polymer or added to the aqueous medium. Hydration rate can also be controlled by the level of applied shear, with the solution viscosity increasing faster when subjected to high shear. The rate of viscosity development may be influenced, particularly in low shear applications, by the salts present in the solution. The extent of retardation of hydration is dependent on the concentration and type of salt. Finally, the viscosity level achieved at a particular point in time is a function of polymer concentration.
Unmodified guar will develop viscosity in all electrolyte systems such as those contained in KCl, NaCl, and CaCl.sub.2 at high concentrations. Guar gum hydrates most efficiently in the pH range of 7 to 8, yielding viscosities of 32-36 cps at 500 sec.sup.-1 in 2% KCl. Guar will not hydrate in organic solvents such as methanol.
Hydroxypropyl guar (HPG) hydrates well in many salt systems at 80.degree. F., and also develops excellent viscosity at temperatures of 40.degree. F. Depending on the mechanical shear applied, 80-90% of the viscosity can be achieved within ten minutes. Optimum hydration of HPG can be realized at a pH in the range of 4 to 6. HPG also viscosifies mixtures of methanol and 2% KCl in water used typically in a ratio of 50/50.
Carboxymethyl hydroxypropyl guar (CMHPG) hydrates most electrolyte make-up solutions, however, it is more sensitive to these solutions than guar or HPG. CMHPG hydrates well in both cold and warm water.
In a manner similar to the above natural polymers, synthetic polymers may also be dispersed and hydrated. However, in contrast to these natural polymers, hydration and dispersion will rely more on mechanical mixing of synthetic polymers.
Several attempts have been made over the last thirty years to perfect the process and chemicals for continuous preparation well treatment fluids. A continuous process would allow the fluids to be made in "real time" during the treatment process. This process would have several advantages over the current common method of producing fluids which involve "batch" mixing of water, gelling agent and other additives into individual "frac" tanks before the treatment is begun. The process is expensive because of the time and equipment required and because of wasted and unused fluid resulting from treatment delays, termination of the treatment before pumping all fluids, and fluid left in the bottom of the tanks which cannot be pumped out. The disposal of unused gelled fluids has also become an expensive process because of stricter laws on the disposal of chemical wastes. More recently, it has been proposed to effect the hydration of a gelable fluid for well treatment operations by increasing the residence time of the gelable fluid in a flow-through operation by providing a series of vertical flow tanks. The hydratable gel material is mixed with water at the beginning of the series of tanks and, in theory, the mixture passes through the series of vertical flow tanks in a "plug flow" which gives the gelable material sufficient time to hydrate in the aqueous mixture. Such a system is described in U.S. Pat. No. 4,828,034 in order to achieve substantially complete hydration of the hydratable gel. However, such system requires the application of high shear such as by pumping the mixture through a centrifugal pump at some point along the series of vertical flow tanks. As used in this specification, the term vertical flow tanks will be understood to mean a series of underflow and overflow tanks wherein the primary flow through the tank is in the vertical direction, up or down.
As used in this specification, the term "substantially complete hydration" shall be understood to mean hydration of a polymer which achieves a viscosity in the range of at least 80-90% of the final viscosity of a completely hydrated gel.
Further, as used in this specification, the term "plug flow" shall be understood to mean any type of flow conditions or associated equipment that tend to simulate a first-in-first-out, FIFO, behavior thus maximizing the effective residence time per unit volume of tank at any given flow.